The present invention relates generally to non-contact gauging systems. More particularly, the invention relates to an apparatus system and method for calibrating non-contact gauging systems.
Demand for higher quality has pressed manufacturers of mass produced articles, such as automotive vehicles, to employ automated manufacturing techniques that were unheard of when assembly line manufacturing was first conceived. Today, robotic equipment is used to assemble, weld, finish, gauge and test manufactured articles with a much higher degree of quality and precision than has been heretofore possible. Computer-aided manufacturing techniques allow designers to graphically conceptualize and design a new product on a computer workstation and the automated manufacturing process ensures that the design is faithfully carried out precisely according to specification. Machine vision is a key part of today""s manufacturing environment. Machine vision systems are used with robotics and computer-aided design systems to ensure high quality is achieved at the lowest practical cost.
Achieving high quality manufactured parts requires highly accurate, tightly calibrated machine vision sensors. Not only must a sensor have a suitable resolution to discern a manufactured feature of interest, the sensor must be accurately calibrated to a known frame of reference so that the feature of interest may be related to other features on the workpiece. Without accurate calibration, even the most sensitive, high resolution sensor will fail to produce high quality results.
In a typical manufacturing environment, there may be a plurality of different non-contact sensors, such as optical sensors, positioned at various predetermined locations within the workpiece manufacturing, gauging or testing station. The workpiece is placed at a predetermined, fixed location within the station, allowing various predetermined features of the workpiece to be examined by the sensors. Preferably, all of the sensors are properly positioned and should be carefully calibrated with respect to some common fixed frame of reference, such as a common reference frame on the workpiece or at the workstation.
Maintaining sensors which are properly positioned and calibrated presents several challenges. In a typical manufacturing environment sensors and their associated mounting structures may get bumped or jarred, throwing the sensor out of alignment. Also, from time to time, a sensor needs to be replaced, almost certainly requiring reorienting and recalibrating. Quite simply, sensor positioning, alignment and calibration requires careful attention in the typical manufacturing plant.
Proper sensor positioning, alignment and calibration can present significant time and labor requirements. For a given part or assembly, the entire manufacturing assembly line may need to be shut down and the workstation cleared, so that the sensor may be positioned, aligned and recalibrated. In some instances this entails placing a highly accurate, and very expensive full-scale model of the part or assembly into the workstation. This independently measured part is sometimes called a master part. The master part is placed in careful registration with the external coordinate system of the workstation and then each sensor is trained on its assigned feature (such as a hole or edge). Once positioned, the sensors are locked into place and calibrated and the master part is removed. Only then can the assembly line be brought back online.
As an alternative to using a master part, it is possible to calibrate the gauging sensor by attaching a target to the sensor and illuminating the target using a plane of structured light produced by the sensor. A pair of optical sighting devices, such as theodolites, are placed at different vantage points within the workspace. The theodolites triangulate on the illuminated target to provide an independent reading of the position of the target. The theodolites are placed at carefully prescribed locations relative to an external reference frame. With the gauging sensor projecting structured light onto the target, the theodolites are manually aimed at the illuminated targets and readings are taken. The respective readings of the theodolites and the gauging sensor are coordinated and translated to calibrate the sensor relative to the external reference frame. It is a trial and error process. If the sensor needs to be reoriented (as is often the case), the theodolites must be manually retrained on the target after each sensor position adjustment. For more information on this calibration technique, see U.S. Pat. No. 4,841,460 to Dewar et al.
Whereas both of the aforementioned calibration techniques do work, there is considerable interest in a calibration technique that is more efficient and easier to accomplish, and which eliminates the need to rely on expensive master parts. To this end, the present invention provides a calibration system that can be used in a matter of minutes, instead of hours, and without the need for precisely manufactured master parts. One of the major advantages of the invention is that it allows the calibration of the sensors to be checked or realigned between line shifts, without requiring the line to be shut down for an extended period.
The calibration system employs reference indicia that are disposed in fixed relation to the external reference frame of the manufacturing or assembly zone or gauging station. A target calibration device is positioned at a vantage point, typically above the gauging station, so that the reference indicia are within the field of view of the target calibration device. The target calibration device is operative to determine the spatial location and orientation of a portable reference target within the gauging station. Exemplary target calibration devices may include, but are not limited to a photogrammetry system, a theodolite system, or a laser tracker system.
The calibration system further employs a portable reference target that is placed within the observation field of the target calibration device and also within the sensing zone of the feature sensor. The presently preferred portable reference target is a three-dimensional framework that provides at least three non-coplanar reflective structures (e.g., straight edges) that can be illuminated by structured light emanating from the feature sensor. As part of the present invention the feature sensor includes, but is not limited to, a structured light triangulation sensor. Although the non-coplanar reflective structures provide the feature sensor with spatial data for measuring the position and orientation of the portable reference target, the present invention improves the accuracy of the measurement data by adapting the target to support a visible dot pattern or a light sensitive imaging array device (e.g., CCD). In this way, the portable reference target provides unambiguous spatial data for measuring its spatial position and orientation.
The calibration system further includes a coordinate transformation system for coordinating the measurement data from the target calibration device and from the feature sensor. More specifically, the calibration system is adapted to collect data from the target calibration device and the feature sensor. The transformation system establishes a first relationship between the reference frame of the target calibration device and the external reference frame. The transformation system also establishes a second relationship between the reference frame of the target calibration device and the reference frame of the feature sensor. Finally, the transformation system determines a third relationship between the reference frame of the feature sensor and the external reference frame, whereby the feature sensor is calibrated with respect to the external reference frame.
The system and technique of the present invention allows for simplified calibration of a feature sensor. The target calibration device is first calibrated via the reference indicia to the external reference frame. Next, the portable reference target is placed within the field of view of the target calibration device and the feature sensor. The portable reference target is calibrated with respect to the reference frame of the target calibration device. The feature sensor is then calibrated by projecting structured light from the feature sensor onto the portable reference target. The structured light intersects the target, producing reflected light patterns at the edges of the target that are then read by the feature sensor. The coordinate transformation system simultaneously receives measurement data as to where the structured light strikes the dot patterns or the light sensitive imaging array devices associated with the target. The coordinate transformation system then performs the appropriate coordinate transformation to map the data of the feature sensor back to the external reference frame. The entire calibration sequence can be performed quite quickly.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.